66 WARREN N. THAYER 



The eastern boundary of this province begins at the Inter- 

 national Boundary and runs in a general southerly direction through 

 the states Chihuahua, Durango, Zacatecas, San Luis Potosi, and 

 Queretaro to the Valle de Mexico. It is not a sharp line like the 

 other boundaries of the province, but has been rather arbitrarily 

 chosen throughout the greater part of its length, though not without 

 certain criteria. It has been drawn to pass near such cities as La 

 Junta, Parral, San Miguel, and Zacatecas, which are known to be 

 situated at the eastern foot of the mountains. Between these 

 cities the line has been drawn to pass through the points marking 

 the sources of the small "suicidal" streams which rise near the 

 border of the mountains and lose their small quantities of water by 

 evaporation and percolation soon after reaching the desert below. 

 The want of suitable topographic maps covering large areas makes 

 a more accurate drawing of this line impossible. Its location 

 cannot be far from correct, however, for it coincides with the line 

 on the Willis geological map of North America, which separates the 

 mountain country, where the volcanic cap rock is thick, from the 

 desert to the east, where erosion has removed the extrusives and 

 exposed the underlying Cretaceous sediments. 



Topography.— hi a general way the topography of this province 

 may be described as "a succession of narrow and continuous 

 northwest-southeast ridges, with foothills, separated by broad 

 and continuous longitudinal valleys" which grow wider toward 

 the coast. 1 The even skyline which one might expect to result from 

 the uplift and dissection of a peneplain is not everywhere apparent, 

 because of subsequent volcanic flows of uneven thickness ; although 

 viewed over broad areas there is a certain evenness of summits which 

 is apparent in spite of the flows, which were not of the cone-building 

 type, but more like those which resulted in the formation of our 

 Columbia Plateau. 



An east-west profile of the Sierra Madre in southern Chihuahua 

 and Sonora shows an average elevation of about 7,000 feet on the 

 east, rising to 10,500 feet at the Continental Divide, and dropping 

 again to about 6,800 feet at the western scarp. 2 The term "Conti- 

 nental Divide" should not be understood too literally, for it is not 



1 J. G. Aguilera, Inst. Geol. Mex., V. 2 Weed, op. cit. 



